Previously, an electromagnetic sound producing device was configured, as shown in FIG. 1, by anchoring a pole piece 2 to the center of case 1, around which an excitation coil 3 is wound. Magnets 4 are placed in the case 1, and a diaphragm 6, having armatures 5 fastened in the center such that they face the top of the pole piece 2, is also placed in case 1.
However, with this electromagnetic sound producing device, it was possible to obtain only a single-peak reproducing characteristic having a peak at the resonance point of the required frequencies, and it was not possible to use it as a low frequency vibrating device since very little sound pressure could be obtained at low frequencies.
In another prior art design, illustrated in FIG. 2, a sound producing device was configured by anchoring a magnet 7 on the bottom plate of a case 8, connecting voice coils 9 to diaphragm 10, and positioning the voice coils 9 in the magnetic space 11 produced by the magnet 7. This design has been popularized as a speaker having a broadband reproduction characteristic of 20 Hz to 20 KHz.
However, this design also cannot be used as a low frequency vibrating body since the necessary sound pressure is not obtainable at very low frequencies. In addition, devices of this type inherently provide only a single-peak reproducing characteristic having a peak at the resonance point of the required frequencies. Consequently, it was not possible to use this device as a low frequency vibrating body.
In applications such as radio-activated pagers, it is desirable to provide selective announcement of an incoming page by either an audible signal or by a low frequency vibration that will be perceived by the person carrying the pager, but not by other persons in the area. In a modern miniaturized pager, space is at a premium. Therefore, the inventor has found that there is a need for a single, compact low-power device that is capable of selectively producing either an audible signal, a low frequency vibration, or both depending on the signals provided to the device.